In contrast to virtual reality, which immerses the user in a wholly computergenerated perceptual environment, augmented reality systems superimpose virtual entities on the user's view of the real world. This concept promises to fulfil new applications in a wide range of fields, but there are some challenging issues to be resolved. One issue relates to achieving accurate registration of virtual and real worlds. Accurate spatial registration is not only required with respect to lateral positioning, but also in depth. A limiting problem with existing optical-see-through displays, typically used for augmenting reality, is that they are incapable of displaying a full range of depth cues. Most significantly, they are unable to occlude real background and hence cannot produce interposition depth cueing. Neither are they able to modify the real-world view in the ways required to produce convincing common illumination effects such as virtual shadows across real surfaces. Also, at present, there are no wholly satisfactory ways of determining suitable common illumination models with which to determine the real-virtual light interactions necessary for producing such depth cues. This thesis establishes that interpositioning is essential for appropriate estimation of depth in augmented realities, and that the presence of shadows provides an important refining cue. It also extends the concept of a transparency alpha-channel to allow optical-see-through systems to display appropriate depth cues. The generalised theory of the approach is described mathematically and algorithms developed to automate generation of display-surface images. Three practical physical display strategies are presented; using a transmissive mask, selective lighting using digital projection, and selective reflection using digital micromirror devices. With respect to obtaining a common illumination model, all current approaches require either . prior knowledge of the light sources illuminating the real scene, or involve inserting some kind of probe into the scene with which to determine real light source position, shape, and intensity. This thesis presents an alternative approach that infers a plausible illumination from a limited view of the scene.